This invention relates to an apparatus and method for accomplishing heat transfer using microfluidic systems, such as may be useful for transferring heat to and from electronic devices and components thereof, including but not limited to computers, analytical instruments, lasers, other similar electronic instruments and apparatuses.
At the outset, it should be noted that although the invention is described with reference to electronic equipment, particularly computers, the invention can be used in practically any application requiring heat transfer.
Many electrically-powered systems include environments having heat-producing elements contained within substantially enclosed areas. For example, within a typical computer system, the heat generated internally by certain components (such as integrated circuit (IC) devices, power supplies, motors, and transformers) may be substantial. This heat must be dissipated at a rate sufficient to maintain critical system components at acceptable temperatures in order to prevent premature component or system failure.
While small increases in operating temperature may not be immediately detrimental to the operation of electronic components, long-term operation at elevated temperature levels may adversely impact their longevity and reliability. This is particularly true for relatively sensitive integrated circuit or data storage devices, such as central processing units (CPUs) or disk drives, respectively, which may suffer disproportionate reductions in longevity with incremental increases in operating temperature. Moreover, the operating speed and reliability of many electronic systems suffers as the operating temperatures of their components rise. Additionally, mechanical effects resulting from wide variations in temperature and high peak temperatures may eventually induce component failures.
A conventional approach for providing heat transfer in computer systems is to cool the air inside a computer enclosure using a fan, which draws cold outside air into the enclosure. Because temperature-sensitive components such as ICs and disk drives typically act as significant heat sources, however, often this conventional approach is not sufficient to maintain air within the computer chassis at a temperature sufficiently below the maximum allowable temperature for these components, since each component may not dissipate heat effectively enough to maintain its temperature within acceptable limits. Additional localized (e.g. forced) cooling of these components often must be used to increase the rate of heat transfer.
Moreover, rising processor speeds and higher capacity circuits cause even greater thermal loads, thus requiring even more cooling air flow through enclosures to dissipate the attendant heat. In the past, conventional design wisdom has dictated that this increased cooling air flow be achieved by increasing the size and/or number of cooling fans within the housing. This, of course leads to several well-known and heretofore unavoidable problems, limitations and advantages such as: increased cost and complexity of the overall electronic apparatus; diminished space within the housing for additional operating components such as printed circuit boards, drives, etc.; increased operating fan and air movement noise; increased interference from electromagnetic radiation; and increased power consumption.
Specifically, as system electronics become more powerful, absent substantially increased cooling flow these electronic components can dissipate so much heat that they can create localized xe2x80x9chot spotsxe2x80x9d within an enclosure that make it very difficult to maintain thermal compliance for all of the electronics. Making matters worse, consumers have become more sensitive to noise and continue to demand quieter computers and electronic devices. This demand from consumers contradicts the need to increase air flow through the system to maintain thermal compliance. Efforts to improve thermal compliance issues have met with little success due to the rapid increase of electronic component power dissipation. Safety agencies limit the area that can be vented from a computer housing due to potential fire hazards. Thus, in the past, as electronic power dissipation has increased with the rise in processor speeds, so has cooling fan speed and size. Elevated fan speeds have resulted in decreased fan life, and the noise created by this approach has resulted in consumer complaints.
One approach for enhancing heat removal from localized areas or individual components such as ICs uses heat sinks, typically composed of a series of metal fins, that may be attached to component, preferably in close physical contact. Use of a heat sink permits (1) a larger heat transfer area to be used, and (2) heat to be drawn away from the component to another area. For example, a heat sink contacting an integrated circuit may project directly into a stream of cooling air such as may be provided by fans mounted on the walls of the case or on top of the heat sink. A forced flow of cooling air absorbs the heat from the heat sink and the resulting warmed air is blown outside the computer case.
A passive heat sink coupled with a fan, however, is often insufficient to provide adequate local cooling. The heat sink is typically placed next to a fan positioned to blow air out of the enclosure. Since air typically enters the enclosure case from a side opposite the fan, the air is usually pre-heated by other components within the system before reaching the heat sink to a level higher than the ambient air temperature. This can lower the cooling efficiency to such a point that the desired component (e.g., an integrated circuit) coupled to the heat sink may not get an appropriate amount of cooling. Since the desired component may not be totally cooled, the excess heat is typically dissipated throughout the enclosure causing the temperature of the enclosure to rise. In such systems the temperature of the enclosure may be up to 10 degrees Celsius higher than the ambient air temperature.
Moreover, certain electrical components used to increase computing speeds and characterized by particularly high power dissipation have required larger and larger heat sinks to keep the air flow requirements to reasonable levels. These heat sinks have grown to very large sizes in the past several years in order to keep fan speeds to a minimum. Many recent heat sink designs have exceeded the dimensional limits of the socket to which the electrical device is connected. This has caused problems for factories producing the systems, the designers of the system boards, and the end users. It has also caused difficulties for consumers seeking to upgrade their systems. For example, most modern microprocessors are designed to fit into a socket having an integral handle (e.g., a zif socket). To permit a processor to be inserted into, removed from, and locked within such a socket, the handle must be free to rotate up and down. Most heat sinks associated with these processors have exceeded the dimensional limits of the socket, thus causing the handle to be inaccessible. As a result, removing the processor becomes an onerous task once the heat sink is installed.
Another traditional approach to satisfying ever-increasing heat dissipation requirements has been to dramatically increase the size of an air inlet to an enclosure. Such inlets are commonly placed along the front plastic bezel of an enclosure. Increasing the size of an air inlet helps to increase air flow, but only to a certain degree since there is a limit as to how much the front side vents can be enlarged. Industrial designers generally desire to keep such inlets from becoming an eyesore for the consumer, and the visible vent area on a front bezel hampers the industrial designer""s ability to provide a clean design that is aesthetically pleasing. Additionally, safety agencies refuse to approve or xe2x80x9clistxe2x80x9d enclosures having air inlets sized so large as to permit a user""s finger to enter the device.
Therefore, there exists a great need for improved cooling systems for electronic equipment and similar devices. It would desirable to provide a solution for localized heating of components within electronic equipment that would cool the hottest components without dissipating the heat to other components. If accomplished, this should result in the further benefit of reducing the temperature of the enclosure surrounding the electronics due to reduced heat retention. Additionally, it would be desirable for a new cooling system to be characterized by low noise and also to be substantially free of electromagnetic interference or vibrations that might detrimentally impact system performance. Furthermore, it would be desirable for a new cooling system to be characterized by low power consumption, particularly when used in conjunction with battery-powered devices. The desirability of reducing power consumption ins particular acute in devices such as laptop computers, since air-cooling fans in such devices may consume up to 35% of the total available power, thus severely limiting battery life.
Microfluidic heat exchangers and heat exchange systems utilizing an internal working fluid and capable of transferring heat between high and low temperature regions are provided to perform functions such as cooling heat generating components of electronic equipment, computers, lasers, analytical instruments, medical equipment and the like. In such systems, heat is preferably transferred from a heat source to a heat sink using a circulating heat transfer fluid.
In a first separate aspect of the invention, a microfluidic heat exchange system for cooling an electronic component internal to a device such as a computer is provided. The heat exchange device is substantially in interfacial contact with a heat-generating electronic component and supplies an internal operating fluid a heat exchange zone. Operating fluid flows into the heat exchange zone at a first fluid temperature that is lower than the component temperature, and then exits the zone at a second fluid temperature higher than the first fluid temperature.
In a second separate aspect of the invention, a heat exchange system for affecting the temperature of an electronic component includes: a microfluidic heat exchanger having an inlet and an outlet wherein fluid enters at a inlet temperature and exits at an outlet temperature; a fluid circulation device for causing the fluid to flow through the exchanger; and a heat transfer device for returning the operating fluid to the heat exchanger at the inlet temperature.
In a further separate aspect of the invention, an electronic apparatus such as a computing device includes a housing or enclosure and a microfluidic heat exchange system.
In a further aspect of the invention, any of the foregoing aspects may be combined for additional advantage.
These and other aspects and advantages of the invention will be made apparent by reviewing the appended drawings and detailed description.
Definitions
The term xe2x80x9cmicrofluidicxe2x80x9d as used herein is to be understood, without any restriction thereto, to refer to structures or devices through which fluid(s) are capable of being passed or directed, wherein one or more of the dimensions is less than 500 microns. Microfluidic devices according to the present invention typically comprise channels, chambers, and/or reservoirs containing fluids for accomplishing heat transfer. Construction of microfluidic devices is described in co-pending applications, U.S. patent application Ser. Nos. 09/1550,184 and 09/453,029, the entire contents of which are incorporated herein by reference. Such disclosures are also provided in two WIPO PCT patent applications, nos. PCT/US00/27366 and PCT/US00/27313, which were published on Apr. 12, 2001.
The terms xe2x80x9cchannelxe2x80x9d and xe2x80x9cchamberxe2x80x9d as used herein is to be interpreted in a broad sense. Thus, they are not intended to be restricted to elongated configurations where the transverse or longitudinal dimension greatly exceeds the diameter or cross-sectional dimension. Rather, such terms are meant to comprise cavities or tunnels of any desired shape or configuration through which liquids may be directed. Such a fluid cavity may, for example, comprise a flow-through cell where fluid is to be continually passed or, alternatively, a chamber for holding a specified, discrete amount of fluid for a specified amount of time. xe2x80x9cChannelsxe2x80x9d or xe2x80x9cchambersxe2x80x9d may be filled or may contain internal structures comprising valves or equivalent components.
The term xe2x80x9cstencilxe2x80x9d as used herein refers to a preferably substantially planar material from which one or more variously shaped and oriented portions are cut (or otherwise removed) through the thickness of the material layer to form microstructures. The outlines of the cut or removed portions comprise the lateral boundaries of microstructures that are formed by sandwiching one or more stencil layers between other stencils and/or substrates.
The term xe2x80x9cviaxe2x80x9d refers to a fluidic passage that permits fluid to flow between non-adjacent layers of a microfluidic device. A simple via may include an aperture defined in a device layer that is sandwiched between other layers. A via is preferably aligned with one or more fluidic channels, chambers, or other vias. A via may be smaller than, larger than, or the same size as channels or vias defined in one or more adjacent device layers.